1. Field of the Invention
This invention relates to an optical recording method, and more particularly to an optical recording method which is useful for a laser printer.
2. Discussion of the Related Art
FIG. 11 is a diagram showing the configuration of a conventional optical recording apparatus. In FIG. 11, a laser beam 2 emitted from a laser light source 1 is modulated by an acoustooptic light modulator (hereinafter abbreviated as "AO light modulator") 4. Then, the modulated laser beam is directed through a scanning lens 7 by a rotary polygon mirror 6 to a photoconductive drum 9, so as to optically scan the drum 9 in the direction of arrow 10. To the AO light modulator 4, optical recording information is supplied from a computer 137 through a control circuit 13 and a light modulator driving circuit 12. On the basis of the optical recording information, the AO light modulator 4 optically modulates the intensity of the laser beam. A photodetector 8 detects part of the scanning light and transmits a synchronizing signal to the control circuit 13. The photoconductive drum 9 rotates in the direction of arrow 11 (i.e., in the sub-scanning direction), so that optical recording is effected by the laser beam on the entire surface of the photoconductive drum 9.
In general, optical recording information supplied from the computer 137 is recorded at recording positions arranged at regular pitches in both the main scanning and sub-scanning directions. Thus, when an oblique straight line which is closely perpendicular to scanning lines 15 is to be printed, the resultant printed line is irregular or in the form of a series of steps, exhibiting poor printing quality, as shown in FIG. 12(A). In FIG. 12(A), reference numeral 14 designates optically recorded dots, and 16 designates recording pitches in the main scanning direction. When the pitches in the main scanning and sub-scanning directions are small, high-quality printing can be attained. However, the provision of such small pitches increases the volume of the information for printing, thereby increasing the cost of the entire system. In one of the methods which solve this problem, when an oblique straight line closely perpendicular to the scanning lines 15 is to be printed, the control circuit itself checks the contents of original information, and then controls the recording timing to be delayed or advanced, based on the judgment of the control circuit, so that the recording positions are shifted in the main scanning direction, thereby attaining a smooth and straight oblique line as shown in FIG. 12(B). This method is effective in recording an oblique straight line closely perpendicular to the scanning lines.
However, the above method is not effective in recording an oblique straight line closely parallel to the scanning lines 15, as shown in FIG. 13. More specifically, the recorded line is irregular in the form of a series of steps. Thus, the method involves a problem that there is no possibility of improvement in printing quality.
In order to solve this problem, Japanese Laid-open Patent Publication No. Hei 3-131818 has proposed an image forming apparatus in which, when a laser beam is deflected in the main scanning direction, the laser spot is simultaneously shifted every other dot by a minute distance in the sub-scanning direction, thereby increasing the dot density in the sub-scanning direction and reducing the degree of irregularity or step difference in the oblique line to be recorded.
In the proposed apparatus, however, a laser spot is always shifted every one dot in the sub-scanning direction, and therefore the dot density in the scanning direction is reduced, and such an effect that step difference in the oblique line is reduced may become small or lowered.